In ST2 I use a customized version of Slush & Poppies, but in ST3 it gives a cryptic "Bad XML" error. In fact this happens even if I take the Slush & Poppies from /opt/sublime_text/Packages/Color Scheme - Default.sublime-package, copy it to ~/.config/sublime-text-3/my.tmTheme, put in the appropriate "color_scheme": ... line in my preferences file and save.

Saw another possibly related post on this. Thinking out loud, it could be that a bug has crept in to Sublime's XML parser, or perhaps, if Sublime is using python for this, the bug is with the newer python's XML parser.

This is likely from S3 no longer opening files from full path names, instead it expects the file to be located under the Packages directory, such as in Packages/User (accessible via Preferences/Browse Packages)